The present invention relates to a low friction bearing arrangement and, more particularly, to such an arrangement in which a rotatable shaft is supported for free rotation by a quantity of ferrofluid.
A ferrofluid is a ferromagnetic fluid which may be magnetically polarized. The fluid is substantially uniform, and has the property that when a gradient magnetic field is applied to it, a body force is developed within the fluid which can substantially exceed the force of gravity applied to the fluid. Typically, a ferrofluid comprises a colloidal dispersion of finely divided magnetic particles of subdomain size whose liquid condition is unaffected by the presence of an applied magnetic field. The ferrofluid particles typically range in size up to about 300 A, the remain uniformly dispersed throughout the liquid carrier due to thermal agitation. Ferrofluids are discussed in U.S. Pat. No. 3,917,538, issued Nov. 4, 1975, to Rosensweig.
A principal application of ferrofluids in the past has been for use as low friction seals. In such a sealing arrangement, a shaft made of a magnetically permeable material defines one or more annular ridges which directly oppose an outer, annular pole piece. Magnetic flux is applied to a flux path including the shaft and the pole piece. The flux bridging the gaps between the ridges and the pole piece holds rings of ferrofluid in position therebetween. These rings of ferrofluid act as seals and preclude the passage of gas along the shaft.
Numerous bearing designs have been utilized in the past in various applications for supporting a rotatable shaft. Even with the best of such designs, however, the frictional forces resisting shaft movement may be greater than desired. Further, bearing wear may occur over extended periods of use. It is seen, therefore, that an improved bearing arrangement is needed in which a shaft may be mounted for rotation at extremely low frictional levels, and in which bearing life is extended.